Abstract
When an oxygen-saturated acetonitrile (MeCN) solution containing the NADH model compound, 9,10-dihydro-l0-methylacridine (AcrH2), is irradiated with u.v. light firstly in the presence and secondly in the absence of perchloric acid (HClO4), AcrH2 is oxidized by oxygen to yield the 10-methylacridinium ion (AcrH+) and 10-methyl-9-acridone (AcrO) respectively, and reduction of oxygen yields hydrogen peroxide. The u.v. irradiation of a neutral aqueous solution containing 1-benzyl-1,4-dihydronicotinamide (BNAH) also results in the oxidation of BNAH by oxygen to yield BNA+ and H2O2. Kinetic studies and detection of radical intermediates by e.s.r. spectroscopy have revealed that the photo-oxidation of NADH model compounds by oxygen proceeds via radical-chain reactions, which are initiated by electron transfer from the singlet excited state of NADH model compounds to oxygen. The chain carrier for the photo-oxidation of AcrH2 in the presence of HClO4 in MeCN as well as for the photo-oxidation of BNAH in a neutral aqueous solution is hydroperoxy radical (HO2˙), while acridinylperoxy radical (AcrO2˙) acts as a chain carrier for the photo-oxidation of AcrH2 in the absence of HClO4, in MeCN.Although no photo-oxidation of AcrH2 by oxygen occurs under visible-light irradiation, AcrH2 is efficiently oxidized to AcrH+ in the presence of a flavin analogue used as a sensitizer, in MeCN under otherwise identical conditions. The flavin-sensitized photo-oxidation of AcrH2 by oxygen proceeds via one-electron reduction of a flavin analogue by AcrH2 followed by the efficient oxidation of the reduced flavin by oxygen without the appreciable contribution of radical-chain processes.
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